1. Field of the Invention
The present invention relates to the field of electrochemical devices which include at least one electrochemically active layer which is capable of reversibly and simultaneously inserting ions and electrons, in particular electrochromic devices. These electrochemical devices are specially used to manufacturer glazings whose light and/or energy transmission or whose light reflection may be modulated by means of an electric current. They may also be used to manufacture energy storage elements such as batteries or gas sensors or display elements.
2. Description of the Background
Considering the particular example of electrochromic systems, it will be recalled that the latter include a layer of a material capable of reversibly and simultaneously inserting cations and electrons and whose oxidation states, corresponding to the inserted state and to the ejected state, have a distinct color, one of the states generally being transparent. The insertion or deinsertion reaction is controlled by a suitable power supply, especially by applying a suitable potential difference. The electro-chromic material, which is generally based on tungsten oxide, must thus be brought into contact with a source of electrons such as a transparent electroconductive layer, and source of cations, such as an ionically conductive electrolyte.
Moreover, it is known that to ensure at least a hundred switching operations, a counterelectrode must be associated with the layer of electrochromic material, this counterelectrode also being capable of reversibly inserting cations, symmetrically with respect to the layer of electrochromic material so that, macroscopically, the electrolyte appears as a simple cation medium.
The counterelectrode must consist either of a layer which is neutral in color or at least transparent when the electrochromic layer is in the decolored state. Since tungsten oxide is a cathodic electrochromic material, that is to say that its colored state corresponds to the most reduced state, an anodic electrochromic material such as nickel oxide or iridium oxide is generally used for the counterelectrode. It has also been proposed to use a material which is optically neutral in the oxidation states in question, such as, for example, cerium oxide or organic materials like the electroconductive polymers (polyaniline, etc.) or Prussian blue.
Such systems are described, for example, in European Patent Nos. 0 338 876, 0 408 427, 0 575 207 and 0 628 849.
Currently, these systems may be grouped into two categories, depending on the type of electrolyte that they use:
(i) The electrolyte is in the form of a polymer or of a gel, for example a polymer exhibiting proton conduction, such as those described in European Patent Nos. 0 253 713 and 0 670 346, or a polymer exhibiting conduction of lithium ions such as those described in European Patent Nos. 0 382 623, 0 518 754, or 0 532 408; PA1 (ii) The electrolyte is an inorganic layer which is ionically conductive but electronically insulating (one speaks of "all-solid" electrochromic systems).
All these electrochemical devices allow satisfactory reversibility of the ion insertion/deinsertion phenomena and, therefore, of the coloration/decoloration phenomena in the specific case of electrochromic systems. However, it seems that this reversibility character tends to degrade over time, especially because of prolonged exposure to ultraviolet rays, or to heat (for example when the temperature reaches 80.degree. C.), or because of a large number of switching operations from one coloration state to another.
This problem has already been studied in the aforementioned European Patent No. 0 628 849. This patent proposes a first solution consisting of interposing between the electrolyte and the counterelectrode, a layer called a "barrier layer" which is permeable to the ions which it should reversibly insert/deinsert and which will limit the degradation of the system by retarding the irreversible reduction of the counterelectrode, or indeed its dissolution, in contact with the electrolyte. However, this solution has its limits, because, given the nature, resulting mainly from the method of manufacture, of the electroconductive layers which underlie the deposition of the electrochemically active layers, and mainly their significant roughness, it is observed that the barrier layer must be relatively thick in order to effectively fulfil the role of protecting the counterelectrode which is devolved thereon. Now, the drawback of a thick barrier layer resides in a partial or even total loss of the functionality of the entire system, or of part of the system; that is to say that it slows down or even suppresses the reversible ion insertion/deinsertion reactions at one or both of the electrochemically active layers.